Resistance Spot Welding Process with Pulsed Current Pattern to Improve Joint Strength of Ultra High Strength Steel Sheets

We have developed a new resistance spot welding process with “pulsed current pattern”, which consists of short-time high-current post-heating and short-time cooling to improve joint strength of ultra high strength steel (UHSS) sheets with a tensile strength over 980MPa. The high-current post-heating reheats the heat affected zone (HAZ) near the electrodes and that near the sheet-sheet interface rather than the center of the nugget, and this pulsed current pattern utilizes this procedure to improve the toughness of the nugget. In the case of 980MPa grade steel with a 1.6mm thickness, the pulsed current pattern improved cross tension strength (CTS) from 7.2kN to over 10kN and its failure mode from interface failure to plug failure. Tensile shear strength (TSS) showed no drop due to the pulsed current pattern, while the conventional temper pattern welding decreased TSS by about 3kN. 1 time pulsed current achieved CTS above 10kN with plug failure with nugget diameters of 5.2mm to 4.5mm, and 2 time pulsed current expanded the lower limit to 4.4mm. CTS over 10kN and plug failure can be obtained by pulsed current pattern in cases of 980MPa and higher grade steel.